Hot-rolled steel sheet having excellent drawability and post-processing surface hardness

ABSTRACT

The present invention intends a hot-rolled steel sheet with a sheet thickness in a range of 2 to 15 mm, having component composition including C: 0.3% or less by mass %, Si: 0.5% or less, Mn: in a range of 0.2 to 1%, P: 0.05% or less, S: 0.05% or less, Al: in a range of 0.01 to 0.1%, N: in a range of 0.008 to 0.025%, and the remainder being iron and unavoidable impurities. The hot-rolled steel sheet includes an amount of solute N: not less than 0.007%, and ferrite grains existing at a position of t/4 in depth (t: the sheet thickness) includes the ferrite grains, a sheet plane orientation thereof, within 10° from the (123) plane, having an area ratio: not less than 20%, the ferrite grains, a sheet plane orientation thereof, within 10° from the (111) plane, having an area ratio: not less than 5%, and the ferrite grains, a sheet plane orientation thereof, within 10° from the (001) plane, having an area ratio: 20% or less, the average grain size of the ferrite grains existing at the position of t/4 in depth being in a range of 3 to 35 μm.

TECHNICAL FIELD

The invention relates to a hot-rolled steel sheet exhibiting a predetermined surface hardness after processing, while exhibiting excellent drawability during the processing.

BACKGROUND ART

In an attempt for improvement in automobile fuel efficiency from the viewpoint of environmental protection, there has lately increasingly risen the demand for lower weight, that is, higher strength of a steel for use in various automobile parts including transmission parts, such as a gear and a case. In order to respond to requirements for the lower weight·the higher strength described as above, a steel obtained by subjecting a bar steel to hot forging (a hot forging steel) has since been adopted as a steel in general use. Further, in order to attain abatement of CO₂ emission in a process of manufacturing parts, there has also increasingly risen the demand for a switch to manufacturing parts, such as a gear, etc., by cold forging although those parts have so far been worked on by hot forging.

Now, cold forming (the cold forging) has an advantage over hot working and wet working in that the cold forming is not only higher in productivity, but also excellent in both size-accuracy and steel yield, as compared with the hot working and the wet working. However, a problem posed in the case of manufacturing parts by use of the cold forming is the necessity of inevitably using a steel that is high in strength, that is, high in deformation resistance. Unfortunately, there exists a drawback in that the higher deformation resistance of a steel in use is, the shorter will be the service life of a mold for the cold forming, but also more susceptible to cracking the mold will be at the time of the cold forming.

For this reason, there was on occasions adopted a traditional method, whereby a steel is formed into a predetermined shape by use of the cold forging, and subsequently, heat treatment, such as quenching, tempering, etc., is applied thereto, thereby manufacturing high-strength parts having a predetermined strength (hardness) as ensured. However, because part dimensions inevitably undergo a change during the heat treatment after the cold forging, there was the need for correcting the part dimensions by machining, such as secondary cutting, etc., and therefore, a problem-solving measure was highly desired whereby both the heat treatment, and subsequent working can be omitted.

It is disclosed that, in the case of, for example, a low-carbon steel, progress in natural aging is controlled by making use of solute C to ensure a predetermined age-hardening amount due to strain aging in an attempt to solve the problem described as above, whereupon a cold forging wire rod·bar steel, having excellent strain-aging properties, can be obtained (refer to Patent Literature 1).

However, this technique being intended to control the strain aging by virtue of a solute C amount only, it has been difficult to obtain a steel having compatibility with both sufficient cold formability, and post-processing hardness·strength as required.

Accordingly, the present applicant focused attention on a difference between respective influences of solute C, and solute N, contained in a steel, exerted on deformation resistance and static strain aging hardening, respectively, having carried out various studies. As a result, the present applicant has found that if the respective amounts of these solid-solutionized elements are properly controlled, it is possible to obtain a steel for use in machine construction, exhibiting a predetermined surface hardness (strength) after the cold forming (the cold forging), while exhibiting excellent cold formability during the cold forming, and the present applicant has already submitted an application for patent (refer to Patent Literature 2).

With this steel, there was realized compatibility with both cold formability, and higher hardness (higher strength) after the cold forming, however, the steel has a drawback in that a production cost was high because the steel was a hot forging steel, as was the case of the wire rod·bar steel, described in Patent Literature 1. Accordingly, in order to achieve further reduction in the production cost, manufacturing of an automobile parts by the cold forming of a hot-rolled steel sheet substituting for a traditional hot forging steel is also under study.

For example, a hot-rolled steel sheet for nitriding, capable of obtaining a high surface-hardness and a sufficient hardening-depth, after the nitriding, has been proposed (refer to Patent Literature 3).

However, this technique has a problem in that sufficient reduction in the production cost cannot be realized because further nitriding after the cold forming is required of the technique.

Further, there has been proposed a hot-rolled steel sheet having chemical composition including C: 0.10% or less, Si: less than 0.01%, Mn: 1.5% or less, and Al: 0.20% or less, together with (Ti+Nb)/2: in a range of 0.05 to 0.50%, S: 0.005% or less, N: 0.005% or less, O: 0.004% or less, and the total of S, N, and 0: 0.0100% or less, while not less than 95% of the microstructure thereof being effectively a ferrite single-phase microstructure. This hot-rolled steel sheet is described to be excellent in dimensional accuracy in a precision punching plane, very high in the surface hardness of a punching plane after punching, and furthermore, excellent in resistance to red-scale defect as will. (refer to Patent Literature 4).

However, with this hot-rolled steel sheet, N, which is regarded as a deleterious element, is limited to an extremely low content, and accordingly, this hot-rolled steel sheet is utterly different in technical principles from the hot-rolled steel sheet according to the applicant's invention, making positive use of N.

In general, with respect to control of the texture, for enhancement in formability of a steel sheet, it has been traditionally clarified on both experimental and theoretical basis that the lager a plastic anisotropy (r value (Lankford value): a ratio of a sheet-width strain to a sheet-thickness strain, in a tensile test) of a material is, the higher will be the deep drawing workability of a thin steel sheet for use in the outer panel of an automobile body, and further, with a recrystallization texture, it is essential for enhancement in deep drawing workability to strongly develop the (111) plane in parallel with a sheet plane orientation, while weakening the (100) plane orientation (refer to Nonpatent Literature 1).

For this reason, efforts to deal with enhancement in the formability, through the control of the texture of a steel sheet, has variously been tried.

There has been proposed, for example, a high-strength thin steel sheet, which is a steel, sheet having chemical composition including C: in a range of 0.01 to 0.1% (hereinafter % denotes mass %), Si: in a range of 0.01 to 2%, Mn: in a range of 0.05 to 3%, P≦0.1%, S≦0.03%, Al: in a range of 0.005% to 2.0%, N≦0.01%, and B in a range of 0.0005% to 0.0030%, together with Ti in a range satisfying Ti-(48/12)C-(48/14)N-(48/32)S≧−0.03%, the remainder including Fe and unavoidable impurities, while a value obtained by dividing hardness variations of the steel sheet by the mean value thereof is not more than 0.2, and a plane strength of the {110} plane in the rolling direction is not more than 1.7, the high-strength thin steel sheet being excellent in blanking ring-forging properties (refer to Patent Literature 5).

Further, there is proposed, a high-strength steel sheet having chemical composition including C: in a range of 0.0005 to 0.10% (hereinafter % denotes mass %), Si: 1.5% or less, Mn: in a range of 0.1 to 3.0%, P: 0.080% or less, S: 0.03% or less, sol, Al: in a range of 0.01 to 0.50%, and N: 0.005% or less, together with one element or two elements selected from the group consisting of Nb: 0.20% or less, and Ti: 0.20% or less, and the remainder including Fe and unavoidable impurities. And not less than 60% of the steel microstructure of the high-strength steel sheet, in volume fraction, is a ferrite phase, and with respect to three-dimensional crystal orientation density function ODF {φ1, φ, φ2}, ODF {0°, 0°, 45° } at a time when φ is 0°, φ1 is 0°, and φ2 is 45° has a strength 3.0 or less, and ODF {0°, 35°, 45°} at a time when φ1 is 0°, and φ2 is 45° has a strength in a range of 2.5 to 4.5, thereby enabling a high-strength steel sheet small in ductility-anisotropy, and insusceptible to cracking at the time of press forming to be provided (refer to Patent Literature 6).

There has been proposed a hot-rolled steel sheet having chemical composition including C: in a range of 0.01 to 0.05% (hereinafter % denotes mass %), Si: 0.2% or less, Mn: 0.50% or less, Al: in a range of 0.005 to 0.10%, P: 0.05% or less, S: 0.05% or less, N: 0.01% or less, and O: 0.01% or less, the remainder including Fe and unavoidable impurities, and having a microstructure which is a ferrite single phase with an average grain size in a range of 40 to 60 μm, while a random strength ratio of the planes {118}, <110>, and that of the planes {115}, <110> are each not less than 10, and a random strength ratio of the planes {111}, <110>, and that of the planes {111} to <121> are each not more than 1, thereby enabling a hot-rolled steel sheet small in respect of in-plane anisotropy after cold rolling-recrystallization to be provided (refer to Patent Literature 7).

Although the various attempts for enhancement in the formability of a steel sheet, through the control of the aggregate structure, have been made, as shown in those examples described as above, those attempts are mostly concerned with the outer panel of an automobile body, automobile body-frames, and automobile suspension-parts. Formability required of the transmission parts, such as the gear, and the case, etc., as the target of the present invention, differ from formability required of the automobile body parts, and therefore, the former is a part of which not only deep drawing workability, and ironing are required, but also the surface hardness after working is required.

Furthermore, in the field of the transmission parts, progress is being made in studies on switching of a method, from manufacture of parts by use of forging (hot forging, cold forging, etc.) toward manufacture of the transmission parts, using a steel sheet, in an attempt to lower the weight as well as the cost of the part, however, further improvement in formability is required.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application Publication No. H10 (1998)-306345,

Patent Literature 2: Japanese Unexamined Patent Application Publication No. 2009-228125,

Patent Literature 3: Japanese Unexamined Patent Application Publication No. 2007-162138,

Patent Literature 4: Japanese Unexamined Patent Application Publication No. 2004-137607,

Patent Literature 5: Japanese Unexamined Patent Application Publication No. 2009-24226,

Patent Literature 6: Japanese Unexamined Patent Application Publication No. 2012-158797,

Patent Literature 7: Japanese Unexamined Patent Application Publication No. 2007-291514

Nonpatent Literature

Nonpatent Literature : Iron and Steel Institute of Japan: “Recrystallization·Texture, Application to Texture Control, Forefront of Recrystallization Studies”, March, 1999, p. 208

SUMMARY OF INVENTION Technical Problem

The present invention has been developed by focusing attention on the circumstances described as above, and it is an object of the invention to provide a hot-rolled steel sheet exhibiting a predetermined surface hardness after processing, while exhibiting excellent drawing workability during the processing.

Solution to Problem

According to embodiment 1 of the present invention, there is provided a hot-rolled steel sheet with a sheet thickness in a range of 2 to 15 mm, excellent in drawing workability and post-processing surface hardness, having component composition including C: 0.3% or less by mass % (the same applies to the following for the chemical components), (except for 0%); Si: 0.5% or less (except for 0%); Mn: in a range of 0.2 to 1%; P: 0.05% or less (except for 0%); S: 0.05% or less (except for 0%); Al: in a range of 0.01 to 0.1%; N: in a range of 0.008 to 0.025%, and the remainder being iron and unavoidable impurities. The hot-rolled steel sheet includes an amount of solute N: not less than 0.007%, and ferrite grains existing at a position of t/4 in depth (t: the sheet thickness, the same applies hereunder), includes the ferrite grains, a sheet plane orientation thereof, within 10° from the (123) plane, having an area ratio: not less than 20%, the ferrite grains, a sheet plane orientation thereof, within 10° from the (111) plane, having an area ratio: not less than 5%, and the ferrite grains, a sheet plane orientation thereof, within 10° from the (001) plane, having an area ratio: 20% or less, the average grain size of the ferrite grains existing at the position of t/4 in depth being in a range of 3 to 35 μm.

According to embodiment 2 of the invention, the component composition of the hot-rolled steel sheet described in embodiment 1 may further include 2% or less (except for 0%) of Cr and/or 2% or less (except for 0%) of Mo.

According to embodiment 3 of the invention, the component composition of the hot-rolled steel sheet described in embodiment 1 or 2 may further include at least one element selected from the group consisting of Ti: 0.2% or less (except for 0%), Nb: 0.2% or less (except for 0%), and V: 0.2% or less (except for 0%).

According to embodiment 4 of the invention, the component composition of the hot-rolled steel sheet described in any one of embodiments 1 to 3 may further include B: 0.005% or less (except for 0%).

According to embodiment 5 of the invention, the component composition of the hot-rolled steel sheet described in any one of embodiments 1 to 4 may further include at least one element selected from the group consisting of Cu: 5% or less (except for 0%), Ni: 5% or less (except for 0%), and Co: 5% or less (except for 0%).

According to embodiment 6 of the invention, the component composition of the hot-rolled steel sheet described in any one of embodiments 1 to 5 may further include at least one element selected from the group consisting of at least one element selected from the group consisting of Ca: 0.05% or less (except for 0%), REM: 0.05% or less (except for 0%), Mg: 0.02% or less (except for 0%), Li: 0.02% or less (except for 0%), Pb: 0.5% or less (except for 0%), and Bi: 0.5% or less (except for 0%).

Advantageous Effects of Invention

According to the present invention, by securing a solute N amount, and controlling the texture of a hot-rolled steel sheet to a predetermined microstructure form, in a microstructure composed mainly of ferrites having a predetermined average grain size, the service life of a mold can be extended by improving deformability during processing even in the case of an automobile part of which drawing workability is required, and it has become possible to provide a hot-rolled steel sheet insusceptible to occurrence of cracking, and capable of securing a predetermined surface hardness for an automobile part obtained after working on the steel sheet

DESCRIPTION OF EMBODIMENTS

A hot-rolled steel sheet according to the present invention (hereinafter referred to also as “the present invention steel sheet” or simply as “the steel sheet”) is hereunder descried in further details. The present invention steel sheet has a feature in common with the hot forging steel described in Patent Literature 2, the feature being the securing of a solute N amount, however, the present invention steel sheet differs from the latter in that the C-content is permitted up to a scope on a slightly higher side, and the ferrite grain is refined, while the texture form of the steel sheet is controlled.

[Sheet Thickness of the Present Invention Steel Sheet: 2 to 15 mm]

First, the present invention steel sheet having a sheet thickness in a range of 2 to 15 mm is the target of the invention. If the sheet thickness is less than 2 mm, rigidity required of a structure will not be secured. On the other hand, if the sheet thickness is in excess of 15 mm, it will be difficult to achieve the texture form of the steel sheet, as defined in the present invention, so that desired effects of enhancement in drawing workability will no longer be obtained. The sheet thickness is preferably in a range of 3 to 14 mm.

Next, the component composition of the present invention steel sheet is described hereunder. Chemical component is indicated hereinafter in units of mass % with respect to all elements.

[Component Composition of the Present Invention Steel Sheet]

<C: 0.3% or less (except for 0%)>

C is an element exerting a large effect on the formation of the microstructure of the steel sheet, and as the microstructure is a primarily ferrite microstructure, or a ferrite·pearlite double-phase microstructure, C is also an element, the content of which need be limited in order to have the primarily ferrite microstructure including a pearlite fraction as low as possible. If the C-content is excessive, the pearlite fraction in the steel sheet microstructure will increase, thereby raising the risk that deformation resistance will become excessive due to work-hardening of the pearlite. Accordingly, the C-content in the steel sheet is preferably limited to 0.3% or less, preferably to 0.25% or less, more preferably to 0.2% or less, and further preferably to 0.15% or less, in particular. However, if the C content is too low, this will render it difficult to satisfy the strength and the surface hardness, after deep drawing, while causing difficulty in deoxidation during melting of a steel, and the C-content is therefore preferably not less than 0.0005%, more preferably not less than 0.0008%, and further preferably not less than 0.001%, in particular.

<Si: 0.5% or less (except for 0%)>

Si is an element, the content of which need be reduced as much as possible because if Si is solid-solutionized in steel, this will cause an increase in deformation resistance of the steel sheet,. Accordingly, in order to control an increase in the deformation resistance, an Si content in the steel sheet is limited to 0.5% or less, preferably to 0.45% or less, more preferably to 0.4% or less, further preferably to 0.3% or less, in particular. However, if the Si content is extremely low, this will render it difficult to satisfy the strength and the surface hardness after the deep drawing, while causing difficulty in deoxidation during melting of a steel material, and the Si content is therefore preferably set to not less than 0.005%, more preferably not less than 0.008%, and further preferably not less than 0.01%, in particular.

<Mn: in a range of 0.2 to 1%>

Mn is an element having actions for desulfurization and deoxidation, respectively, in a steel-making process. Further, in the case where an N-content in a steel material is increased, cracking becomes susceptible to occurrence due to dynamic strain aging hardening caused by heat generation during processing, however, on the other hand, Mn has an effect of enhancement in workability at that time to thereby suppress cracking. In order to cause these actions to be effectively exhibited, an Mn-content in a steel material is set to not less than 0.2%, preferably not less than 0.22%, and more preferably not less than 0.25%. However, if the Mn content is excessively high, deformation resistance will become too large, causing heterogeneous microstructure due to segregation. The Mn-content is therefore set to 1% or less, preferably to 0.98% or less, and more preferably to 0.95% or less.

<P: 0.05% or less (except for 0%)>

P is an impurity element unavoidably contained in steel, however, if P is contained in a ferrite, P is an element causing an increase in the deformation resistance, because P will be segregated on a ferrite-grain boundary to thereby deteriorate cold formability, and cause solid-solution strengthening of the ferrite. Accordingly, it is desirous to reduce a P-content as much as possible from the standpoint of cold formability, however, since extreme reduction in the P-content will invite an increase in steel-making cost, the P-content is set to 0.05% or less in consideration of processability, and preferably set to 0.03% or less.

<S: 0.05% or less (except for 0%)>

S is an unavoidable impurity element as with the case of P, and further, this is an element that is precipitated as FeS in a film-like state on a grain boundary to thereby deteriorate workability. Further, P also acts to trigger hot brittleness. Accordingly, from the standpoint of enhancement in deformability, an S-content, in the present invention, is set to 0.05% or less, and preferably to 0.03% or less,. However, it is industrially difficult to render the S-content to 0. Further, since S has an advantageous effect of enhancement in machinability, a recommendable S-content is preferably not less than 0.002%, and more preferably not less than 0.006%, from the standpoint of enhancement in machinability.

<Al: in a range of 0.01 to 0.1%>

Al is an element effective for deoxidation in the steel-making process. In order to obtain the effect of the deoxidation, an Al-content in a steel material is set to not less than 0.01%, preferably not less than 0.015%, and more preferably not less than 0.02%. However, if the Al-content is too high, this will cause deterioration in toughness and cracking will become susceptible to occurrence, and therefore, the Al-content is set to 0.1% or less, preferably 0.09% or less, and more preferably 0.08% or less.

<N: in a range of 0.008 to 0.025%>

N is an important element for obtaining a predetermined strength by the agency of post-processing static strain aging hardening. Accordingly, an N-content in a steel material is set to not less than 0.008%, preferably not less than 0.0085%, and more preferably not less than 0.009%. However, if the N-content becomes excessive, the effect of post-processing dynamic strain aging hardening, besides the static strain aging hardening, will become pronounced, thereby increasing deformation resistance, which is unsuitable, and therefore, the N-content is set to not more than 0.025%, preferably not more than 0.023%, and more preferably not more than 0.02%.

<Amount of Solute N: not less than 0.007%>

And, if a predetermined amount of solute N (hereinafter, referred to as “an amount of solute N”) is secured in a steel sheet, this will enable the static strain aging hardening to be stimulated without excessively increasing deformation resistance. In order to secure a desired strength after cold forming, the amount of solute N not less than 0.007% is needed. However, if the amount of solute N becomes excessive, this will cause deterioration in cold formability, and therefore, the amount of solute N is preferably set to not more than 0.03%. In this connection, since the N-content in the steel material is not more than 0.025%, the amount of solute N will never effectively become 0.025% or more.

Herein, the amount of solute N according to the present invention means an amount found by subtracting the total amount of all N-compounds from the total of the N-amounts in a steel material, pursuant to JIS G 1228. A practical method for measuring the amount of solute N is shown hereunder by way of example.

(a) An Inert Gas Melting Method—A Thermal Conductivity Method (Measurement on the Total of the N-Amounts)

A sample cut out of a specimen is put in a melting pot to be melted in an inert-gas flow, whereupon N is extracted. Extracted material is transported to a heat-conductivity cell for measurement on variations in heat conductivity to thereby find the total of the N-amounts.

(b) An Ammonia Distillation-Separation Indophenol Blue Absorptionmetric Method (Measurement on the Total Amount of All N-Compounds)

A sample cut out of a specimen is dissolved in a 10% AA base electrolyte to apply constant-current electrolysis thereto, thereby making a measurement on the total amount of all N-compounds. The 10% AA base electrolyte in use is a nonaqueous solvent-based electrolyte made up of 10% acetone, 10% TMA chloride, and methanol as the remainder, which is a solution capable of preventing formation of a passive film on the surface of a steel.

About 0.5 g of the sample taken from the specimen is dissolved in the 10% AA base electrolyte, and insoluble residues (N-compounds) as generated are filtrated through a polycarbonate filter provided with pores 0.1 μm in pore size. The insoluble residues thus obtained are heated within sulfuric acid, potassium sulfate, and pure copper chips for decomposition, and decomposed substances are joined with a filtering liquid. This solution is turned alkaline by use of sodium hydrate to be subsequently subjected to steam distillation, whereupon distillated ammonia is absorbed into dilute sulfuric acid. Further, phenol, sodium hypochlorite, sodium pentacyanonitrosylferrate (III) are added thereto to cause a blue complex to be generated, thereby making a measurement on absorbance by use of a absorptiometer, whereupon the total amount of all N-compounds is found.

Then, the amount of solute N can be found by subtracting the total amount of all N-compounds found by the method under (b) described as above from the total of the N-amounts found by the method under (a) described as above.

The present invention steel sheet basically includes the components described in the foregoing, and the remainder includes iron and unavoidable impurities, however, the following tolerable components may be added thereto.

<Cr: 2% or less (except for 0%) and/or Mo: 2% or less (except for 0%)>

Cr is an element having an action of raising a strength on a grain boundary to thereby enhance deformability and in order to cause such an action as described to be effectively exhibited, a Cr-content is preferably not less than 0.2%. However, if the Cr-content is excessive, this will raise the risk of an increase in deformation resistance to thereby deteriorate cold formability, and it is therefore recommendable that the Cr-content is preferably set to not more than 2%, more preferably not more than 1.5%, and further preferably not more than 1%, in particular.

Further, Mo is an element having an action of increasing the hardness as well as the deformability of a post-processing steel, and in order to cause such an action as described to be effectively exhibited, a Mo-content is therefore preferably set to not less than 0.04%, and more preferably not less than 0.08%. However, if the Mo-content is excessive, this will raise the risk of deterioration in cold formability, and therefore, it is recommendable that the Mo-content is preferably set to not more than 2%, more preferably not more than 1.5%, and further preferably not more than 1%, in particular.

<At least one element selected from the group consisting of Ti: 0.2% or less (except for 0%), Nb: 0.2% or less (except for 0%), and V: 0.2% or less (except for 0%)>

These elements each are an element having high affinity for N to be able to coexist with N to form N-compounds, thereby grain-refining a steel, enhancing toughness of a product obtained after cold forming, and enhancing resistance to cracking properties. However, even if the content of each of these elements exceeds an upper limit value, the effects of improvement in the respective properties cannot be grained. It is therefore recommendable that the content of each of those elements is set to 0.2% or less, preferably in a range of 0.001 to 0.15%, and more preferably in a range of 0.002 to 0.1%, in particular.

<B: 0.005% or less (except for 0%)>

As is the case of Ti, Nb, and V, respectively, described as above, B is an element having high affinity for N to be able to coexist with N to form N-compounds, thereby grain-refining a steel, enhancing toughness of the product obtained after the cold forming, and enhancing the resistance to cracking properties. For this reason, if the present invention steel sheet contains B, the amount of solute N as desired can be secured to thereby enhance the strength after cold forming, it is therefore recommendable that a B-content is set to 0.005% or less, preferably in a range of 0.0001 to 0.0035%, and more preferably in a range of 0.0002 to 0.002%, in particular.

<At least one element selected from the group consisting of Cu: 5% or less (except for 0%), Ni: 5% or less (except for 0%), and Co: 5% or less (except for 0%)>

These elements each are an element effective for enhancing post-processing strength as the element has an action of causing a steel to undergo strain-aging to be thereby hardened. In order to cause such an action as described to be effectively exhibited, the content of each of these elements is set to not less than 0.1%, and preferably not less than 0.3%. However, if the content of each of those elements is excessive, this will arise the risk that the effects of not only strain aging and hardening, but also enhancement in the post-processing strength will be saturated, and cracking is fostered. It is therefore recommendable that the content of each of those elements is preferably set to 5% or less, more preferably 4% or less, and more preferably 3% or less, in particular.

<At least one element selected from the group consisting of Ca: 0.05% or less (except for 0%), REM: 0.05% or less (except for 0%), Mg: 0.02% or less (except for 0%), Li: 0.02% or less (except for 0%), Pb: 0.5% or less (_(ex)cept for 0%), and Bi: 0.5% or less (except for 0%)>

Ca is an element causing spheroidizing of a sulfide-compound based inclusion, such as MnS, etc., to thereby improve deformability of steel, and contributing to enhancement in machinability. In order to cause such an action of this element, as described, to be effectively exhibited, a Ca-content is preferably set to not less than 0.0005%, and preferably to not less than 0.001%. However, even if the Ca-content is excessively high, the effect thereof will be saturated, and an advantageous effect commensurate with the Ca-content cannot be expected, it is therefore recommendable that the Ca-content is set to 0.05% or less, preferably 0.03% or less, and more preferably 0.01% or less, in particular.

As in the case of Ca, REM is an element causing spheroidizing of a sulfide-compound based inclusion, such as MnS, etc., to thereby contribute to enhancement in machinability, while improving deformability of steel. In order to cause such an action of this element, as described, to be effectively exhibited, an REM-content is preferably set to not less than 0.0005%, and more preferably not less than 0.001%. However, even if the REM-content is excessively high, the effect thereof will be saturated, and an advantageous effect commensurate with the REM-content cannot be expected, it is therefore recommendable that the REM-content is set to not more than 0.05%, more preferably not more than 0.03%, and further preferably not more than 0.01%, in particular.

Further, in the present invention, REM means Lanthanoid elements (15 elements from La up to Ln), Sc (Scandium) and Y (Yttrium). At least one element selected from the group consisting of La, Ce, and Y, among these elements, is preferably contained in REM, and La and/or Ce are more preferably contained therein.

As in the case of Ca, Mg is an element causing spheroidizing of a sulfide-compound based inclusion, such as MnS, etc., to thereby contribute to enhancement in machinability, while improving deformability of steel. In order to cause such an action of this element, as described, to be effectively exhibited, an Mg-content is preferably set to not less than 0.0002%, and more preferably not less than 0.0005%. However, even if the Mg-content is excessively high, the effect thereof will be saturated, and an advantageous effect commensurate with the Mg-content cannot be expected, it is therefore recommendable that the Mg-content is preferably set to not more than 0.02%, more preferably not more than 0.015%, and further preferably not more than 0.01%, in particular.

As is the case with Ca, Li is an element causing spheroidizing of a sulfide-compound based inclusion, such as MnS, etc., to improve deformability of steel, and render an Al-base oxide harmless by lowering the melting point thereof, thereby contributing to enhancement in machinability. In order to cause such an action of this element, as described, to be effectively exhibited, an Li-content is preferably set to not less than 0.0002%, and more preferably not less than 0.0005%. However, even if the Li-content is excessively high, the effect thereof will be saturated, and an advantageous effect commensurate with the Li-content cannot be expected, it is therefore recommendable that the Li-content is preferably set to 0.02% or less, more preferably 0.015% or less, and further preferably 0.01% or less, in particular.

Pb is an element effective for enhancement in machinability. In order to cause such an action of this element, as described, to be effectively exhibited, a Pb-content is preferably set to not less than 0.005%, and more preferably not less than 0.01%. However, if the Pb-content is excessively high, this will cause occurrence of a structural problem, such as a rolling defect, etc., it is therefore recommendable that the Pb-content is preferably set to 0.5% or less, more preferably 0.4% or less, and further preferably 0.3% or less, in particular.

As is the case with Pb, Bi is an element effective for enhancement in machinability. In order to cause such an action of this element, as described, to be effectively exhibited, a Bi-content is preferably set to not less than 0.005%, and more preferably not less than 0.01%. However, even if the Bi-content is excessively high, the effect of the enhancement in machinability will be saturated, it is therefore recommendable that the Bi-content is preferably set to 0.5% or less, more preferably 0.4% or less, and further preferably 0.3% or less, in particular.

Next, there is described hereinafter a texture which is the feature of the present invention steel sheet.

[The Texture of the Present Invention Steel Sheet]

The present invention steel sheet is based on a steel of the primarily ferrite microstructure, or the ferrite-pearlite double-phase microstructure, as described in the foregoing, however, the present invention steel sheet is characterized in that a sheet plane orientation of the ferrite grain, and the size thereof are each controlled to a defined scope, in particular.

The texture of the present invention steel sheet is composed of the ferrite-pearlite double-phase microstructure. If pearlite is excessively exists, this will cause deterioration of the formability of the steel sheet, and therefore, pearlite, in terms of area ratio, is preferably set to 10% or less, more preferably to 9% or less, and further preferably to 8% or less, in particular, while the remainder being ferrite.

<With respect to ferrite grains existing at a position of t/4 in depth (t: sheet thickness, the same applies hereunder),

an area ratio of the ferrite grains, the sheet plane orientation thereof being within 10° from the (123) plane: not less than 20%,

an area ratio of the ferrite grains, the sheet plane orientation thereof being within 10° from the (111) plane: not less than 5%, and

an area ratio of the ferrite grains, the sheet plane orientation thereof being within 10° from the (001) plane: 20% or less>

A form of the texture varies by a processing method even in the same crystal system, to be expressed in respect of a rolling plane, and the direction of rolling, in the case of a rolling stock. In the present invention, the rolling plane is expressed as the plane (◯◯◯). Herein, ◯ denotes an integer number. Expression of each of these orientations is described in, for example, “Texture” (published by Maruzen Co. Ltd.), edited and written by Shinichi Nagashima.

With the present invention, with respect to the ferrite grains existing at the position of t/4 in depth, the deep drawing formability of the hot-rolled steel sheet can be enhanced by controlling the area ratio of the ferrite grains, the sheet plane orientation thereof being within 10° from the (123) _(p)lane to not less than 20%, the area ratio of the ferrite grains, the sheet plane orientation thereof being within 10° from the (111) plane to not less than 5%, and the area ratio of the ferrite grains, the sheet plane orientation thereof being within 10° from the (001) plane to 20% or less, respectively.

It has been traditionally known that, with respect to the sheet plane orientation of the ferrite grain, if the (001) plane orientation is weakened, while strongly developing the (111) plane orientation parallel with the sheet plane, this will be effective for enhancement of the deep drawing workability. In the case of a steel making process where a cold rolling step and an annealing step are applied, respectively, it has been possible to execute such a control of the sheet plane orientation, as described above, however, in the case of a hot rolling process and a hot rolled steel sheet having a steel thickness ranging from 2 to 15 mm, which is large as a thin steel sheet, it has been difficult to execute such a control of the sheet plane orientation, as described above.

Accordingly, with the present invention, the ferrite grain having the sheet plane orientation of the (123) plane is newly introduced so as to enable texture control in the hot rolled steel sheet, having thereby succeeded in realization of enhancement in formability.

The ferrite grain having the (123) plane as the sheet plane orientation, as described above, has an action of enhancing formability and deep drawing workability, and in order to cause the ferrite grains to effectively exhibit such an action, these ferrite grains need to be not less than 20% in the area ratio, preferably not less than 22%, more preferably not less than 24%, and further preferably not less than 26%, in particular.

The ferrite grains having the (111) plane as the sheet plane orientation, as well has an action of enhancing formability and deep drawing workability, and in order to cause the ferrite grains to effectively exhibit such an action, these ferrite grains need to be not less than 5% in the area ratio, preferably not less than 6%, and more preferably not less than 8%.

The (001) plane causes occurrence of in-plane anisotropy due to forming, thereby deteriorating formability. Accordingly, the area ratio of these ferrite grains is limited to 20% or less, preferably to 18% or less, and more preferably to 15% or less.

Further, the microstructure form of the hot-rolled steel sheet has microstructure distribution in the direction of the sheet-thickness, and the microstructure form is defined such that a position at the depth t/4, (t: the sheet-thickness) is a representative position. Further, the ferrite grains having the sheet plane orientation within 10° from the respective ideal plane orientations ((123) plane, (111) plane, and (001) plane) are considered to have a substantially equivalent action, and therefore, the ferrite grains having the sheet plane orientation in respective scopes are defined to have the respective area ratios described as above.

<The average grain size of the ferrite grains existing at the position t/4, in depth: 3 to 35 μm>

The average grain size of the ferrite grains making up the ferrite microstructure need be in a range of 3 to 35 μm in order to enhance the workability (drawing workability, bending workability, and press workability) of the steel sheet, and satisfy post-processing surface texture. If the ferrite grain is excessively fine, the deformation resistance will become too high, and therefore, the average grain size of the ferrite grains is set to not less than 3 μm, preferably not less than 4 μm, and more preferably not less than 5 μm. On the other hand, if the ferrite grain is excessively bloated, this will cause deterioration in toughness, and fatigue property, etc., and even if the crystal orientation is controlled, press formability, such as bending workability, overhanging, etc., will noticeably deteriorate, so that failures, such as cracking at the time of forming, and rough surface, etc., are susceptible to occurrence. Accordingly, the average grain size of the ferrite grains is set to 35 μm, or less, preferably 30 μm or less, and more preferably 28 μm or less. Further, with the hot-rolled steel sheet, there exists a size distribution of the ferrite grains in the direction of the sheet-thickness, as is the case with the microstructure form, and the average grain size of the ferrite grains was defined by adopting the position of the respective ferrite grains, at the depth t/4, as the representative position.

[Method for Measuring the Area Ratios of Respective Phases]

With reference to the area ratios of respective phases, respective specimen steel sheets were subjected to nital corrosion, and 5 visual-fields of a specimen is photographed by use of an scanning electron microscope (SEM; magnification 1000×), whereupon respective ratios of ferrite and pearlite can be found by a spot counting method.

[Method for Measuring the Sheet Plane Orientation of the Ferrite Grain]

Measurement·analysis of the sheet plane orientation of the ferrite grain is conducted by use of SEM-EBSP (Electron Back Scattering Pattern) and EBSD (Electron Back Scattering Diffraction). For an SEM apparatus, use is made of, for example, an SEM (JEOLJSM5410) manufactured by Nippon Denshi Co. Ltd, and for an EBSP measurement·analysis system, use is made of, for example, an EBSP (OIM): manufactured by TSL Corp., respectively. Further, a specimen measurement-region is set to (300 to 1000 μm)×(300 to 1000 μm), and a measurement-step interval is set to, for example, a range of 1 to 3 μm. On the basis of the crystal orientations of the respective ferrite grains thus identified, the ferrite grains having the orientation within 10° from the respective ideal plane orientations were tabularized to find the total area, whereupon the total area is divided by an area of each of the measurement-regions, having thereby found an area ratio for every ideal plane orientation.

[Method for Measuring the Average Grain Size of the Ferrite Grains]

With respect to the average grain size of the ferrite grains, the maximum diameters of the respective ferrite grains observed in each of predetermined measurement-regions were measured by use of SEM-EBSP described as above, and measurement-condition thereof, and a mean value of measurement values was found as the average grain size.

Then, there is described hereunder a preferable production method for obtaining the present invention steel sheet described as above.

[Preferable Production Method for the Present Invention Steel]

For production of the present invention steel, any method may be adopted if a steel sheet in a desired sheet-thickness can be formed from a steel material having the component composition described in the foregoing by use of such a method. For example, molten steel having the component composition described as above is adjusted in a converter under the following condition to be tuned into a slab by use of a process for ingot-making, or continuous casting, and subsequently, the slab is rolled into a hot-rolled steel sheet in sheet-thickness as desired, thereby enabling the present invention steel to be produced.

[Adjustment of the Molten Steel]

An N-content in the molten steel can be adjusted by adding a raw material containing an N-compound to the molten steel at the time of melting in the converter, and/or by controlling the atmosphere of the converter so as to in an N₂ atmosphere.

[Heating]

Heating prior to hot-rolling is executed in a range of 1100 to 1300° C. In this heating, a heating condition at a high-temperature is required in order to cause N as much as possible to be solid-solutionized without forming N-compounds. The lower limit of a heating temperature is preferably 1100° C., and more preferably 1150° C. On the other hand, the heating temperature in excess of 1300° C. is difficult from an operational point of view.

[Hot Rolling]

Hot rolling is executed such that a finish-rolling temperature will be at 870° C. or higher. If the finish-rolling temperature is excessively low, this will cause occurrence of ferrite transformation at a high temperature, and precipitation carbide in ferrite will be bloated, thereby causing deterioration in fatigue strength. Therefore, the finish-rolling temperature not lower than a certain temperature is required. Since the finish-rolling temperature causes austenite grains to be coarsening, while rendering the ferrite grain size to be larger to some extent, the finish-rolling temperature is preferably set to not lower than 900° C. Further, the upper limit of the finish-rolling temperature is set to 1000° C. because as it is difficult to secure temperature.

[Hot Rolling Pass Schedule]

The hot-rolled steel sheet according to the invention has a sheet thickness in a range of 2 to 15 mm, however, in order to refine the ferrite grain size to thereby control the average grain size so as to fall within a predetermined grain size range, there is the need for not only controlling the rolling-temperature described as above, but also keeping the final rolling-reduction ratio not lower than 15% for tandem rolling in the finish-rolling. In the finish-rolling, the tandem rolling with 5 to 7 passes is normally performed, however, as a pass schedule is set from the viewpoint of controlling sheet-gripping, the final rolling-reduction ratio is on the order of from 12 to 13%. The final rolling-reduction ratio is preferably not lower than 16%, and more preferably not lower than 17%. The higher the final rolling-reduction ratio is raised (up to 20% or 30%), the more grain-refining effect will be obtained by the crystal grain, however, the upper limit is defined to be on order of 30% from the viewpoint of rolling control.

[Output Rate of Hot Rolling]

There is also the need for controlling an output rate of the finish rolling in order to obtain the texture of the ferrite grains having the sheet plane orientation described as above, and to control the texture so as to be uniform as much as possible in the direction of the sheet-thickness. For this reason, the output rate for the final pass is controlled to a range of 300 to 700 m/min. If the output rate is either too high or too low, the desired sheet plane orientation tends to become harder to obtain, and the texture is susceptible to being non-uniform in the direction of the sheet-thickness, which is undesirable. Further, if the output rate is low, productivity as well will deteriorate. The output rate for the final pass is preferably in a range of 350 to 650 m/min, and more preferably in a range of 400 to 600 m/min.

[Quenching After Hot Rolling]

Quenching at a cooling rate not less than 20° C./s (a first quenching rate) is executed within 5 s, subsequent to completion of the finish rolling, and the quenching is stopped at a temperature in a range of 580 to 670° C. (a quenching-stop temperature). The reason for this is to obtain the primarily ferrite microstructure, that is, the ferrite·pearlite dual-phase microstructure, in which the pearlite fraction is in a tolerable scope. If a cooling rate (a quenching rate) is less than 20° C./s, this will stimulate pearlite transformation, or if the quenching-stop temperature is below 580° C., this will stimulate the pearlite transformation or bainite transformation, whereupon, in either of both cases, it will become difficult to obtain a ferrite·pearlite steel with the pearlite fraction falling within the tolerable scope, and controlling to the texture as desired cannot be made, so that the drawing workability and bending workability will deteriorate. On the other hand, if the quenching-stop temperature is 670° C. or higher, this will cause the precipitation carbide in the ferrite to be bloated, thereby similarly causing the drawing workability and bending workability to undergo deterioration. The quenching-stop temperature is preferably in a range of 600 to 650° C., and more preferably in a range of 610 to 640° C.

[Slow-Cooling After the Quenching-Stop]

Slow-cooling at a cooling rate at not more than 10° C./s (a slow-quenching rate) is executed by natural cooling, or air cooling, for the duration of 5 to 20 s, after the quenching is stopped. By so doing, the precipitation carbide in the ferrite is suitably refined, while allowing a sufficient progress in formation of the ferrite to be made. If the cooling rate exceeds 10° C./s, or if slow-cooling time is less than 5 s, a formation-amount of the ferrite will become insufficient, and the control to the desired texture cannot be made, thereby causing deterioration in the drawing workability and bending workability. On the other hand, if the slow-cooling time exceeds 20 s, the precipitation carbide will not be bloated, thereby causing deterioration in fatigue strength.

[Quenching After the Slow-Cooling, Coiling ]

Quenching at a cooling rate (a second quenching rate) not less than 20° C./s is executed after the slow-cooling to be followed by coiling at a temperature in a range of 300 to 450° C. The reason for this is to establish the primarily ferrite microstructure to enable the desired texture to be formed in order to secure the drawing workability and bending workability. If the cooling rate (the second quenching rate) is less than 20° C./s, or a coiling temperature is in excess of 450° C., much pearlite will be formed. On the other hand, if the coiling temperature is below 300° C., martensite, and residual-austenite are formed, thereby causing deterioration in the drawing workability and bending workability.

The present invention is described hereunder in more details with reference to working examples, however, it is to be understood that the present invention be not limited thereto and that various changes and modification may be possible as falling within the true spirit and scope of the invention, in light of the teachings described as above, and hereinafter.

Working Examples

Steel having a component composition indicated in Table 1 was melted by a vacuum melting method to be cast into an ingot 120 mm in thickness to be subsequently subjected to hot rolling under conditions indicated in Table 2, having thereby produced a hot-rolled steel sheet. For every tests, there were adopted test conditions under which the cooling rate from the completion of the finish rolling up to a quenching stoppage was not less than 20° C./s, and for cooling after the quenching is stopped, the slow-cooling was executed at cooling rate less than 10° C./s, for the duration of 5 to 20 s.

With respect to a hot-rolled steel sheet thus obtained, an amount of solute N, respective area ratios of phases in the microstructure of the steel sheet, respective sheet plane orientations of the ferrite grains, and the average grain size of the ferrite grains were found by using the respective measuring methods described under the heading “Description of Embodiments”

Further, the drawing workability of the hot-rolled steel sheet, which is a steel sheet on the order of 4 to 10 mm in sheet thickness, was evaluated by a cylindrical-formation test under the condition of a punch size φ100 mm, a punch shoulder 8 mm, a dice dia. φ103 mm, and a die shoulder 8 mm. A ratio of a blank diameter (D) to a cylinder dia. d, namely, (D/d) being a draw ratio, the maximum blank diameter, at which a cylinder can be drawn by one drawing without causing rupture, is expressed as Dmax, and Dmax/d is defined as a limiting drawing ratio LDR (Limiting Drawing Ratio), whereupon LDR was used as an evaluation index. LDR in a range of 2 to 2.1 was ranked as “acceptable”.

Further, a test piece after the formation test was taken out, and the outer sides of a cylindrical part and a flange part, respectively, were visually observed. As a result of a visual observation, the case of fissure occurrence was marked “X,” the case of visible cracking although fissure did not occur was marked “Δ”, the case where cracking did not occur although fine irregularities (wrinkle) was observed was marked “◯”, and the case of observing no wrinkle was marked “⊚”. The test piece marked “⊚” or “◯” was evaluated as acceptable. As to “fissure” and “cracking”, a gap having the maximum width not less than 1 mm is defined as “fissure”, while a gap having the maximum width less than 1 mm is defined as “cracking”, thereby differentiating between these defects.

Still further, hardness of the surface of the flange part, after a drawability test conducted as above, was measured, and post-processing hardness of the surface was evaluated. With respect to each of the test pieces after processing, Vickers hardness was measured by use of a Vickers hardness tester under the condition of a load: 1000 g, measurement position: the center part of the test piece, in section, at the position D/4 (D: the diameter of a part), and test times: 5. The test piece having hardness 250 Hv or higher was ranked as “acceptable”.

These test results are indicated in Table 3. In Table 3, notation “(◯◯◯) plane %” under column “ferrite grain” means an area ratio (unit: %) of ferrite grains whose sheet plane orientation is 10° from the (◯◯◯) plane.

TABLE 1 Component (mass %) Steel [The remainder: Fe and unavoidable impurities] type C Si Mn P S Al N Others a 0.02 0.05 0.40 0.007 0.001 0.025 0.011 — b 0.07 0.06 0.40 0.007 0.001 0.022 0.008 — c 0.07 0.07 0.40 0.007 0.001 0.022 0.023 — d 0.07 0.11 0.30 0.007 0.001 0.023 0.009 — e 0.07 0.39 0.21 0.007 0.001 0.024 0.009 — f 0.10 0.05 0.40 0.007 0.001 0.022 0.010 — g 0.15 0.05 0.40 0.007 0.001 0.024 0.009 — h 0.20 0.05 0.40 0.007 0.001 0.022 0.010 — i 0.26 0.05 0.40 0.007 0.001 0.023 0.009 — j 0.07 0.07 0.40 0.007 0.001 0.025 0.003 — k 0.07 0.07 0.40 0.007 0.001 0.025 0.030 — l 0.32 0.07 0.40 0.007 0.001 0.025 0.008 — m 0.07 0.62 0.40 0.007 0.001 0.025 0.010 — n 0.07 0.07 0.17 0.007 0.001 0.025 0.012 — o 0.07 0.07 1.11 0.007 0.001 0.025 0.011 — p 0.07 0.07 0.40 0.065 0.001 0.025 0.010 — q 0.07 0.07 0.40 0.007 0.058 0.025 0.011 — r 0.07 0.07 0.40 0.007 0.001 0.006 0.012 — s 0.07 0.07 0.40 0.007 0.001 0.108 0.013 — t 0.07 0.07 0.40 0.007 0.001 0.025 0.011 Cr: 0.4, Mo: 0.04 u 0.07 0.07 0.40 0.007 0.001 0.025 0.011 Ti: 0.02, Nb: 0.03 v 0.07 0.07 0.40 0.007 0.001 0.025 0.011 B: 0.001 w 0.07 0.07 0.40 0.007 0.001 0.025 0.010 Cu: 0.05, Ni: 0.14 x 0.07 0.07 0.40 0.007 0.001 0.025 0.011 Ca: 0.003, Li: 0.001 y 0.07 0.07 0.40 0.007 0.001 0.025 0.011 V: 0.02 z 0.07 0.07 0.40 0.007 0.001 0.025 0.009 Pb: 0.001, Bi: 0.05 (—: Additive-free, underlined: off the scope according to the present invention)

TABLE 2 Hot rolling condition Sheet Final rolling- Final-pass Finish- Quenching- thickness Heating reduction output rolling stop Coiling of hot-rolled Production Steel temperature ratio rate temperature temperature temperature steel sheet No. type (° C.) (%) (m/min) (° C.) (° C.) (° C.) (mm)  1 a 1250 16 572 922 623 332 4  2 a 1250 16 593 930 601 370 12    3 * a   1000 * 15 539   775 *   542 * 377 4   4 * a 1250 15 566 885 606 439  18 *   5 * a 1250 18   912 * 898 659 391 4   6 * a 1250   9 * 545 889 606 374 4  7 b 1250 16 527 911 600 378 5  8 c 1250 16 510 878 595 356 5  9 d 1250 16 490 876 601 364 5 10 e 1250 16 505 893 658 344 5 11 f 1250 16 576 890 598 405 5 12 g 1250 16 581 898 630 373 5 13 h 1250 17 504 907 645 348 5 14 i 1250 16 508 909 648 332 5 15 j 1250 17 592 910 606 410 5 16 k 1250 15 507 920 641 325 5 17 l 1250 16 530 895 657 412 5 18 m 1250 16 520 899 636 410 5 19 n 1250 17 570 899 597 376 5 20 o 1250 15 559 888 620 320 5 21 p 1250 15 563 917 625 418 5 22 q 1250 15 594 924 632 366 5 23 r 1250 16 515 891 607 352 5 24 s 1250 15 552 923 603 331 5 25 t 1250 17 593 902 617 425 5 26 u 1250 16 505 891 590 366 5 27 v 1250 16 513 901 580 356 5 28 w 1250 15 580 894 648 412 5 29 x 1250 15 550 930 645 405 5 30 y 1250 16 542 906 602 421 5 31 z 1250 16 545 915 613 411 5 (Underlined: off the scope according to the present invention, * = off a recommendable scope)

TABLE 3 Ferrite grain Post- Average Drawing processing Produc- Amount of (123) (111) (001) grain workability surface Steel Steel tion solute N plane plane plane size Surface hardness No. type No. (mass %) (%) (%) (%) (μm) LDR texture (Hv) Remarks 1 a  1  0.0087 35 14 12 29 2.1 ⊚ 275 Present invention steel 2 a  2  0.0092 31 11 15 23 2.0 ◯ 265 Present invention steel 3 a   3 *  0.0028 16 10 29 22 1.7 ◯ 186 Comparative example steel 4 a   4 *  0.0091 14  4 27 40 1.6 X — Comparative example steel 5 a   5 *  0.0089 18  9 25 13 1.8 ◯ 260 Comparative example steel 6 a   6 *  0.0081 21  8 18 38 1.8 X — Comparative example steel 7 b  7 0.007 32 12 15 25 2.0 ◯ 279 Present invention steel 8 c  8 0.018 36 11 13 24 2.0 ◯ 303 Present invention steel 9 d  9 0.008 29 11 11 22 2.0 ◯ 281 Present invention steel 10 e 10 0.008 30 13 15 22 2.0 ◯ 289 Present invention steel 11 f 11 0.009 29 14 13 21 2.0 ◯ 308 Present invention steel 12 g 12 0.008 28 15 14 14 2.0 ◯ 319 Present invention steel 13 h 13 0.009 27 11 16 12 2.0 ◯ 329 Present invention steel 14 i 14 0.008 26 11 11 11 2.0 ◯ 353 Present invention steel 15 j 15 0.002 30 12 16 22 2.1 ◯ 175 Comparative example steel 16 k 16 0.024 31 11 15 23 1.4 X — Comparative example steel 17 l 17  0.0075 28  7 17  9 1.4 X — Comparative example steel 18 m 18  0.0087 14  6 22 24 1.4 X — Comparative example steel 19 n 19 0.011 30  7 25 23 1.5 ◯ 155 Comparative example steel 20 o 20 0.009 16  5 26 21 1.4 X — Comparative example steel 21 p 21 0.009 14  6 30 26 1.4 X — Comparative example steel 22 q 22 0.009 18  3 31 27 1.4 X — Comparative example steel 23 r 23 0.010 15  7 29 26 1.4 X — Comparative example steel 24 s 24 0.011 18  8 32 24 1.4 X — Comparative example steel 25 t 25  0.0078 29 13 15 24 2.0 ◯ 280 Present invention steel 26 u 26  0.0086 30 12 13 23 2.0 ◯ 283 Present invention steel 27 v 27  0.0087 33 13 12 21 2.0 ◯ 291 Present invention steel 28 w 28  0.0088 31 14 14 18 2.0 ◯ 274 Present invention steel 29 x 29  0.0083 33 15 16 17 2.0 ◯ 265 Present invention steel 30 y 30  0.0083 35 14 13 17 2.0 ◯ 273 Present invention steel 31 z 31  0.0083 36 14 12 23 2.0 ◯ 277 Present invention steel (Underlined: off the scope according to the present invention, * = off a recommendable scope surface texture: “⊚” indicates excellent, “◯” indicates good, “Δ” indicates cracking on the surface, “X” indicates fissure occurrence, —: no measurement was made because of the fissure occurrence, Present invention steel: [LDR = 2.0 to 2.1], [surface texture = ⊚ or ◯], and [post-processing surface hardness ≧250 Hv], Comparative example steel: a steel failing to satisfy the requirements for the present invention steel)

Steel Nos. 1, 2, 7 to 14, 23 to 31 were each produced under the recommended hot-rolling condition by using the steel type satisfying requirements set by the component composition of the present invention, as indicated in Table 3, and as a result, it was confirmed that these steel Nos. each are the present invention steel meeting requirements set by the microstructure of the present invention, both the drawing workability and the post-processing surface hardness meet the standard for “acceptable”, and therefore, the hot-rolled steel sheet exhibiting the predetermined surface hardness (strength) after processing, while exhibiting excellent drawing workability during the processing could be obtained.

In contrast, Steel Nos. 3 to 6, and 15 to 24, each were a comparative example steel failing to meet at least either of the respective requirements for the component composition and the microstructure, set by the present invention, and either the drawing workability, or the post-processing surface hardness of the comparative example steel does not meet the standard for “acceptable”.

For example, with the steel No. 3, the requirements for the component composition are satisfied, however, the heating temperature prior to hot-rolling, the finish-rolling temperature at the time of the hot-rolling, and the quenching-stop temperature after the hot-rolling are each too low, off the recommendable scope, the amount of solute N is insufficient, and the ferrite grains of the (123) plane orientation are insufficient, whereas the ferrite grains of the (001) plane orientation become excessive, so that the steel No. 3 is inferior in both the drawing workability and the surface hardness after processing.

Further, with the steel No. 4, the requirements for the component composition are satisfied, however, the sheet thickness after the hot rolling is too large, off the defined scope, and the ferrite grains of both the (123) plane orientation and the (111) plane orientation are become insufficient, whereas the ferrite grains of the (001) plane orientation become excessive and the ferrite grains become bloated, so that the steel No. 4 is inferior in at least the drawing workability.

Still further, with the steel No. 5, the requirements for the component composition are satisfied, however, the output rate for the final pass at the time of the hot rolling is too high off the recommendable scope, and the ferrite grains of the (001) plane orientation become excessive, while the ferrite grains of the (123) plane orientation are insufficient, so that the steel No. 5 is inferior in the drawing workability.

Yet further, with the steel No. 6, the final rolling-reduction ratio at the time of the hot rolling is too low, off the recommendable scope, although the requirements for the component composition are satisfied, so that the ferrite grains become bloated, and the steel No. 6 is inferior in at least the drawing workability.

Further, with the steel No. 15 (steel type j), an amount of solute N is insufficient because an N-content is too low although the hot-rolling condition is in the recommendable scope, so that the steel No. 15 is inferior in post-processing surface hardness.

Meanwhile, with the steel No. 16 (steel type k), an N-content is too high although the hot-rolling condition is in the recommendable scope, so that the steel No. 16 is inferior in at least the drawing workability.

Still further, with the steel No. 17 (steel type 1), a C-content is too high, and an amount of solute N is insufficient although the hot-rolling condition is in the recommendable scope, so that the steel No. 17 is inferior in post processing surface hardness.

Yet further, with the steel No. 18 (steel type m), an Si-content is too high although the hot-rolling condition is in the recommendable scope, and the ferrite grains of the (001) plane orientation become excessive, while the ferrite grains of the (123) plane orientation become insufficient, so the steel No. 18 is inferior in at least the drawing workability.

Further, with the steel No. 19 (steel type n), the Mn-content is too low although the hot-rolling condition is in the recommendable scope, and the ferrite grains of the (001) plane orientation become excessive, so the steel No. 19 is inferior in both the drawing workability and the post-processing surface hardness.

Meanwhile, with the steel No. 20 (steel type o), the Mn-content is too high although the hot-rolling condition is in the recommendable scope, and the ferrite grains of the (001) plane orientation become excessive, while the ferrite grains of the (123) plane orientation become insufficient, so the steel No. 20 is inferior in at least the drawing workability.

Further, with the steel No. 21 (steel type p), the p-content is too high although the hot-rolling condition is in the recommendable scope, and the ferrite grains of the (001) plane orientation become excessive, while the ferrite grains of the (123) plane orientation are insufficient, so that the steel No. 21 is inferior in at least the drawing workability.

Still further, with the steel No. 22 (steel type q), an S-content is too high although the hot-rolling condition is in the recommendable scope, and the ferrite grains of the (001) plane orientation become excessive, while the ferrite grains of both the (123) plane orientation and the (111) plane orientation are insufficient, so that the steel No. 22 is inferior in at least the drawing workability.

Yet further, with the steel No. 23 (steel type r), an Al-content is too low although the hot-rolling condition is in the recommendable scope, and the ferrite grains of the (001) plane orientation become excessive, while the ferrite grains of both the (123) plane orientation are insufficient, so the steel No. 23 is inferior in at least the drawing workability.

Meanwhile, with the steel No. 24 (steel type s), an Al-content is too high although the hot-rolling condition is in the recommendable scope, and the ferrite grains of the (001) plane orientation become excessive, while the ferrite grains of the (123) plane orientation are insufficient, so the steel No. 24 is inferior in at least the drawing workability.

Suitability of the present invention has been confirmed on the basis of the description given as above.

While the present invention has been described in detail by referring to the specific embodiment of the invention, it is to be understood that various variations and modification will be apparent to those skilled in the art without departing from the spirit or scope of the invention.

This application claims a Convention Priority on Japanese Patent Application No. 2013 - 053564, dated Mar. 15, 2013, which is incorporated herein by reference,

INDUSTRIAL APPLICABILITY

The hot-rolled steel sheet according to the invention can be used for the transmission part, such as the gear, and the case, etc., thereby realizing lower weight·higher strength of these parts. 

1. A steel sheet with a sheet thickness in a range of 2 to 15 mm comprising: C: >0 to 0.3 mass; Si: >0 to 0.5 mass %; Mn: in a range of 0.2 to 1 mass %; P: >0 to 0.05 mass %; S: >0 to 0.05 mass %; Al: in a range of 0.01 to 0.1 mass %; N: in a range of 0.008 to 0.025 mass %, solute N: in an amount not less than 0.007 mass %; and the remainder being iron and unavoidable impurities.
 2. The hot rolled steel sheet according to claim 1, further comprising at least one element selected from at least one of groups (a) to (e): (a) at least one element selected from the group consisting of >0 to 2 mass % of Cr and >0 to 2 mass % of Mo, (b) at least one element selected from the group consisting of Ti: >0 to 0.2 mass %, Nb: >0 to 0.2 mass %, and V: >0 to 0.2 mass %, (c) B: >0 to 0.005 mass %, (d) at least one element selected from the group consisting of Cu: >0 to 5%, Ni: >0 to 5 mass %, and Co: >0 to 5 mass %, and (e) at least one element selected from the group consisting of Ca: >0 to 0.05 mass %, REM: >0 to 0.05 mass %, Mg: >0 to 0.02 mass %, Li: >0 to 0.02 mass %, Pb: >0 to 0.5 mass %, and Bi: >0 to 0.5 mass %.
 3. The steel sheet according to claim 1 that comprises (a) at least one element selected from the group consisting of >0 to 2 mass % of Cr and >0 to 2 mass %.
 4. The steel sheet according to claim 1 that comprises (b) at least one element selected from the group consisting of Ti: >0 to 0.2 mass %, Nb: >0 to 0.2 mass %, and V: >0 to 0.2 mass %.
 5. The steel sheet according to claim 1 that comprises (c) B: >0 to 0.005 mass %.
 6. The steel sheet according to claim 1 that comprises (d) at least one element selected from the group consisting of Cu: >0 to 5 mass %, Ni: >0 to 5 mass %, and Co: >0 to 5 mass %.
 7. The steel sheet according to claim 1 that comprises (e) at least one element selected from the group consisting of Ca: >0 to 0.05 mass %, REM: >0 to 0.05 mass %, Mg: >0 to 0.02 mass %, Li: >0 to 0.02 mass %, Pb: >0 to 0.5 mass %, and Bi: >0 to 0.5 mass %.
 8. The steel sheet according to claim 1, wherein the average grain size of the ferrite grains existing at the position of t/4 in depth range from 3 to 35 μm, where t is the thickness of the hot-rolled sheet, and wherein said ferrite grains existing at a position of t/4 in depth, comprise: ferrite grains, a sheet plane orientation thereof, within 10° from the (123) plane, having an area ratio: not less than 20%, ferrite grains, a sheet plane orientation thereof, within 10° from the (111) plane, having an area ratio: not less than 5%, and ferrite grains, a sheet plane orientation thereof, within 10° from the (001) plane, having an area ratio: 20% or less.
 9. The steel sheet according to claim 1 that is a hot-rolled steel sheet.
 10. A gear or other transmission part made from the steel sheet according to claim
 1. 11. A method for making the steel sheet comprising: producing a molten steel slab, and rolling the slab into a sheet ranging from 2 to 15 mm in thickness; wherein said steel slab comprises: C: >0 to 0.3 mass %; Si: >0 to 0.5 mass %; Mn: in a range of 0.2 to 1 mass %; P: >0 to 0.05 mass %; S: >0 to 0.05 mass %; Al: in a range of 0.01 to 0.1 mass %; N: in a range of 0.008 to 0.025 mass %, solute N: in an amount not less than 0.007 mass %; and the remainder being iron and unavoidable impurities. 